HARRISBURG, Pa. _With polls showing him losing ground, Republican John McCain on Tuesday raised the specter of nuclear war to cast doubt on Democrat Barack Obama's readiness to be president.

At a rally in Pennsylvania's capital, McCain recalled how as a naval aviator he was poised for a bombing run during the Cuban Missile Crisis of 1962.

"I sat in the cockpit on the flight deck of the USS Enterprise off of Cuba. I had a target," he said. "My friends, you know how close we came to a nuclear war. America will not have a president who needs to be tested. I've been tested my friends."

As McCain barnstormed through three stops in suburban Philadelphia and Harrisburg, he also accused Obama of flip-flopping on his favorite team in the World Series.

McCain's new line of attack on his opponent, a first-term senator who hasn't served in the military, came as McCain spent a full day on an uphill effort to overtake Obama in a battleground state. Obama campaigned for a second straight day in another battleground, Florida, where he's edged ahead in polls by two points, on average.

At a nighttime rally in Miami, Obama told the crowd that he'd seen some of a McCain speech in Pennsylvania on TV earlier in the day.

"He has decided to completely make up, just fabricate, this notion that I've been attacking Joe the Plumber. I've got nothing but love for Joe The Plumber," Obama said. "That's why I want to give him a tax cut. Senator McCain's decided that if he can't beat our ideas, he'll make up others and run against those. What we need now is not straw men."

He spent the morning focusing on the economy. He convened an economic summit at a community college campus in Lake Worth to showcase his middle-class tax cut and small-business tax credit proposals.

The economic panel included former Federal Reserve Chairman Paul Volcker; Google chief executive Eric Schmidt; Democratic governors of Ohio, Colorado, New Mexico and Michigan; and a small-business owner who told the audience, "I'm not Joe the Plumber."

Obama told the audience that McCain's desire to continue tax cuts for the wealthy and corporations rather than redirect tax cuts to middle- and lower-income workers would continue a decline in family incomes and home values.

"I heard Senator McCain say that I'm more concerned with who gets your piece of the pie than with growing the pie," Obama said. "But make no mistake about it, after eight years of Bush-McCain economics, the pie is now shrinking . . . It is time to try something new."

Volcker, who was Fed chairman from 1979 to 1987, said he never expected to see U.S. and European bank failures such as those that have occurred this year, and that "We've got to rebuild this system from the ground up so that never again is the whole economy threatened by a default on Wall Street."

With polls showing McCain falling behind in battleground states such as Colorado, Wisconsin and Virginia, his campaign is pushing for a comeback in Pennsylvania, which has 21 electoral votes. McCain's wife, Cindy, made four stops in Pennsylvania Monday, and his running mate Sarah Palin has made multiple visits.

McCain still faces a daunting task. Democrats outnumber Republicans by almost 1.2 million in Pennsylvania. Obama has opened his substantial campaign war chest to flood the Philadelphia and Pittsburgh media markets with radio and television ads.

Obama leads McCain in Pennsylvania by 11 points, according to an average of recent polls published by RealClearPolitics.com.

A new national Ipsos/McClatchy poll Tuesday put Obama up by 8 points, with voters' preference for him strengthening on a range of domestic issues. A new national Pew Research Center poll put Obama up by 14 points. It found voters losing confidence in McCain's judgment, more confident about Obama's qualifications and Palin's unfavorability increasing to 60 percent.

In Pennsylvania, McCain said Obama had changed positions on who he's rooting for in the World Series.

Last weekend in Philadelphia Obama said "since the White Sox are out of it, I'll root for the Phillies now." But Monday in Tampa, introduced by a Rays pitcher, Obama said he's the "unity candidate" and "so when you see a White Sox fan showing the love to the Rays — and the Rays showing some love back — you know we're on to something here."

McCain argued that might be symbolic of Obama telling voters whatever they want to hear.

At a small rally at a factory in Bensalem, Pa., McCain said, "I'm not dumb enough to get mixed up in a World Series between swing states, but I think I may have detected a little pattern with Senator Obama. When he's campaigning in Philadelphia, he roots for the Phillies, and when he's campaigning in Tampa Bay, he shows love to the Rays."

McCain said that's also the "way he campaigns on tax cuts, but then votes for tax increases after he's elected."